Using your throwhand pointer
finger push into the string just above the yo-yo. Once the yo-yo has
hit the string, give it slack so that it sits in the hold of the strings.
This is called a Split Bottom Mount.

Use your throwhand to bring the yo-yo forward
over your non-throwhand pointer finger.

Now use your non-throwhand pointer finger and
push it directly into the two string in front of the yo-yo. Then using
your throwhand pointer pull the string in towards yourself and thread
the yo-yo onto the same string it is sitting on.